At present, all solar sails are carefully folded after construction, placed in a rocket's upper stage, and unfurled in space. Sail films are generally tri-layered. A highly refective aluminum layer (which faces the Sun) is affixed to a plastic substrate. One commonly used substrate is Kapton™. An emissive layer, often chromium, is deposited on the side of the sail directed away from the Sun. Its function is to radiate the heat produced by the small amount of solar flux absorbed by the sail's aluminum front face.
Today's "first-generation" sails are very thin—typically a few microns in thickness. Sail areal mass thickness is in the vicinity of a few grams per square meter. You would obtain a film of this mass if you could squash a few raisins and spread them evenly across your table top! Various structural fittings—beams, spars, and cables—add 30% or more to a solar photon
parachute hollow-body parabolic
FIGURE 13.2 Six solar-photon-sail configurations. Cables and spars are solid lines. Payloads are represented by small solid ovals. Arrows denote sunlight and spacecraft acceleration direction.
sail's mass. Experiments have also been performed with inflatable structural elements.
A number of sail configurations have been suggested, researched, or flown. Some of these are shown in Figure 13.2.
In the parachute sail, sunlight pushes against a sail canopy. Payload is attached to the sail by high-tensile strength cables.
The hollow-body, or pillow, sail is inflatable. Sunlight pushes against the "lower" sail surface, which supports the payload.
A more complex arrangement is the parabolic sail, also called the solarphoton thruster (SPT). Here, sunlight is incident against a large, curved "collector" sail and focused against a small thruster sail. The SPT allows for the possibility of "tacking" at a larger angle against the direction of the photon stream.
square heliogyro spinning disk
In a square (or rectangular) arrangement, the payload is attached with spars to the center of the structure. It is this technology that has been the focus of NASA's recent investments.
NASA recently completed a three-year effort to demonstrate key solar sail technologies from two competing teams. L'Garde Inc., of Tustin, California, developed a solar sail system that employs booms that are flexible at ambient temperatures but "rigidized" at low temperatures. The L'Garde sail uses articulated vanes located at the corners of the square to control the solar sail attitude and thrust direction (Figure 13.3). ATK of Goleta, California, developed a coilable longeron that deploys in space in much the way that a spring-loaded screw is rotated to remove it from an object. Once deployed, the sail is unfurled using a network of cables—in a method analogous to the sailing ships ofyesterday. Both hardware vendors fabricated and tested 10-meter subscale solar sails in the spring of 2004; and 20-meter subscale solar sail deployments were conducted under thermal vacuum conditions in 2005.
A spinning heliogyro has four or more sail blades and spins in the direction normal to sunlight, like a gyroscope. The spinning-disk sail is also stabilized by centrifugal force. Its payload is mounted at the center of the sail and supported by a network of beams and spars.
There are many possible variations on sail configuration. The hoop sail, for instance, combines aspects of the parachute and spinning-disk sails. In a hoop sail, structural support takes the form of a hoop the same radius as the sail film and concentric with the sail. Distributed payload components could be suspended from the hoop.
All of the configurations shown in Figure 13.2 have advantages and disadvantages. For instance, the inflatable hollow-body sail is perhaps the easiest to deploy; however, it is also very prone to micrometeorite damage that might release inflating gas during periods of high acceleration.
Solar-photon sails will be huge by any measure. Interplanetary robotic sails have projected diameters of 40-400 meters. Sails supporting human exploration of Mars will carry heavier payloads and will therefore have diameters in the 1-10 kilometer range. Ultimate, space-manufactured sails used to divert asteroids or for interstellar travel could be 100 kilometers in diameter or larger.
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